Intermediate frequency coupling device



March 24, 1936.

A. CROSSLEY 2,035,439

INTERMEDIATE FREQUENCY COUPLING DEVICE Filed Aug. 24, 19:53

,INVENTOR, ALFRED CEOSSLEY,

ATTORNEY.

Patented Mar. 24, 1936 PATENT OFFICE INTERMEDIATE FREQUENCY COUPLING DEVICE Alfred Crossley, Chicago, Ill., assignor to Johnson laboratories, Inc., Chicago, 111., a corporation of Illinois Application August 24, 1933, Serial No. 686,524

50laims.

This invention relates broadly toimprovements in high-frequency amplifying systems, and more particularly to systems designed to be operated at a fixed frequency, such for example as those used in the intermediate amplifiers of superheterodyne radio receivers. Such systems, in general, include, in addition to the amplifying thermionic relay, a resonant circuit consisting of an inductance coil and a condenser. The condenser is normally of a type having very low losses, and includes means for a small adjustment of the capacity value. The efficiency of the complete design depends to a very large extent upon the inductance coil. 2

A principalobject of this invention is to provide a novel type of inductor for use in such amplifying systems, which, because of its des1gn. will greatly increase the efliciency of the system. Additionally, it is the object of this invention to provide designs for the resonant circuits of such amplifying systems which will be more compact, less expensive, and which can be more easily and completely shielded from the magnetic and electrical effects of adjacent similar circuits.

Circuits of the class here contemplated normally occur in pairs, one resonant circuit being connected to the output terminals of a first thermionic relay, and another circuit being connected to the input terminals of a second thermionic relay. In order to transfer energy from the first resonant circuit to the second, the coils are normally coupled electro-magnetically. Either of the thermionic relays may in certain types of apparatus be arranged so that it does not operate 85 strictly as an amplifier. These variations, however, are not material with respect to the application and scope of the present invention.

In present-day designs, coupling devices of the class here under consideration, normally have air cores. Across each of the coils there is connected a condenser whose capacity is adjustable over a limited range. By suitably designing the windings and by adjustment of the associated condensers, each of the circuits may be tuned to be resonant at the desired frequency. In order to effect the desired transfer of energy from the first circuit to the second circuit, it is essential that the electro-magnetic coupling between the two windings shall be closely of the optimum values. In present designs, it is normal to establish the degree of coupling in the design of the device, so that when the coils are assembled the proper coupling will exist.

It will be understood that optimum coupling is the condition in which maximum voltage is generated in the second resonant circuit. This condition is defined in Bulletin '74 of the United States Bureau of Standards (page 59) as that in which in which w is the angular velocity corresponding to the frequency, M is the mutual inductance determining the coupling, and R1 and R2 are the resistances of the two circuits.

Because of the losses which normally exist in m air-core coils of the type now commonly used, it is not essential to provide means for adjusting the coupling between coils after the unit is assembled. It is usually suflicient to determine experimentally what physical relation between the 15 two coils will give approximately the correct coupling and then to mechanically design the device so as to produce this relation.

In the designs herein described, the losses in the coils themselves are greatly decreased, and it therefore becomes desirable to provide means for accurately adjusting each device to the optimum coupling. The present invention, therefore, includes designs for inductors having greatly increased efficiency, and arrangements whereby two 25 such inductors may be inductively related in a device which includes means for producing an accurate adjustment of the electro-magnetic coupling between the windings.

The increased efliciency of the coils themselves 30 is secured by introducing a ferro-magnetic core. These cores are preferably of a type made by compressing individually-insulated magnetic particles of very small size. The size of the particles which will be most advantageous for use in any 35 particular design, will "depend largely upon the frequency for which the system is being designed. In general, the higher the frequency, the smaller the particles will be. The insulation of the individual particles must be sufficiently complete to 0 produce a very high electrical resistivity in the compressed core body, which will then have very low electrical losses.

In cores of the preferred type, it is readily possible to secure apparent permeability of the 5 order of from 8 to 10. This greatly decreases the size of the coil itself, because less turns are required for any desired value of inductance. This materially decreases the resistance of the winding. Since the losses in the air-core coils of 5 present designs are the chief source of methciency, a reduction in the size of the coil will produce a proportionate decrease in the losses of the system, provided that the losses introduced by the introduction of the ferro-magnetic core are 5 sumciently small. Cores oiv the type which I have described possess this property of extremely low losses, so that the iron-core coils are very much more eflicient than the equivalent air-core coils. Because of the higher efliciency or the ironcore coils, it is possible to use much lower degrees of coupling than are permissible with air-core coils. This results in a still further increase in the electrical efliciency of thedevice. In a highfrequency amplifier, for example, it is possible to get substantially twice the amplification while maintaining the selectivity the same as it would be with air-core coils, or, if desired, it is possible to obtain approximately twice the selectivity while maintaining the same amplification. Similarly, a fifty per cent. increase in both amplification and selectivity can be secured.

The invention will be better understood by reference to the accompanying drawing, which illustrates a possible mechanical embodiment thereof. It will be readily understood by those skilled in the art, that numerous mechanical arrangements may be employed to mount the coils upon their term-magnetic cores, and to provide means for mechanical adjustment to vary the electro-magnetic coupling between the windings.

Referring to the drawing, Fig. 1 is an elevation of a simple high-frequency coupling device in which the magnetic circuit is of the open-core type;

Fig. 2 is a wiring diagram showing how the coupling device shown in Fig. 1 is to be connected in a high-frequencyampliiying system.

Referring to Fig. 1, the device here illustrated consists of a base i of insulating material, preferably of a suitable ceramic. This base i is arranged to be conveniently secured to the chassis upon which the other portions of the complete apparatus are mounted. The device consists of windings 2 and 3 mounted upon compressed form-magnetic cores 4 and 5. The winding or coil 2 may be wound directly upon the core member 5, or may be separately wound and secured to the core member by any suitable means, such for example as cementing. Similarly, the coil 3 is also either wound directly on the core member 5 or is secured thereto.

The coils 2, 3, are of the universal wound type, this type giving lowdistributed capacity and low losses. The coils are wound on a machine known 'as a Universal winding machine, and, as shown in the drawing, have relatively few turns per layer, and more layers than turns per layer, thus giving the coil a radial depth greater than its axial length. Additionally, the operation of the winding machine is such as to space the turns in each layer and to so arrange them that portions of turns in contiguous layers are not parallel but cross at a slight angle to one another. The construction is such as to make a self-supporting winding.

Secured to the core member 5 is a sleeve 6 having a shoulder 6a, and a threaded portion 1. The sleeve 6 may be molded into the core member 5 as an insert, and should be of non-magnetic material, such as brass, in order not to introduce additional losses into the system. The sleeve 5 passes through a hole in the base I and is secured thereto by means of a nut 8.

Passing through the sleeve 6 is a rod 9, which is molded into the core member 4 and which has a threaded portion Hi. This rod 9 is also preferably of non-magnetic material, such as brass. Between the core members 4, 5, and surrounding the rod 9, is a compression spring I I which tends to force the two core members apart. On the threaded portion ID of the rod 9 is a nut l2, by which the core member 4, with its coil 2, may be drawn toward the core member 5, with its coil 3, against the action of the spring ll. The nut l2, therefore, provides means by which the electromagnetic coupling between the two windings may be adjusted.

Mounted upon the base i are small adjustable condensers l3, l4, each of which has an adjustable leaf IB, ll. Passing through these leaves and through the base I, are screws l8, l9, which are provided at their lower ends with nuts 20, 2|, by which the capacity value of the condensers l3, 14, may be adjusted within limits. The terminals of the coil 2 may be connected to the terminals of the condenser l3, and the terminals of the coil 3 may be connected to the terminals of The coil and core assembly may conveniently be enclosed in a shield, not shown, which will be in the form of an open-ended cup of a suitable metal such as copper, arranged to be secured to the base I or directly to the chassis upon which the device is mounted.

The device of Fig. l is of the open-core type, the return path for the flux which threads through the core bodies 4, 5, being through the air. This type has been found to be advantageous for the frequencies now commonly used in the intermediate frequency amplifiers of superheterodyne radio receivers. It will be readily understood, however, that in cases where cores of a closed type are advantageous, the general arrangement of Fig. 1 could still be employed.

It will be noted that the coupling between the coils 2, 3, is varied by altering the effective length of the magnetic circuit. The length of the magnetic circuit is decreased when the core body t, with its coil 2, is drawn closer to the core body 5, with its coil 3, not only because the gap between the core bodies 43, 5, has been decreased, but also because the length of the return path through the air has been equally shortened. Fig. 2 shows a coupling device of the type shown in Fig. 1 suitably connected in a high-frequency amplifying system. The winding 2 has an adjustable condenser l3 connected across its terminals, and one side of the resonant circuit thus formed is connected to the plate 26 of a thermionic relay 21. The other side of the resonant circuit, consisting of the winding 2 and the condenser i3, is connected to a suitable source of voltage supply for the plate 26. The winding 3 has an adjustable condenser l4 connected across its terminals, and one side of the resonant circuit thus formed is connected to the input electrode 28 of a :econd thermionic relay 29. The other side of the resonant circuit, consisting of the coil 3 and the condenser l 4, is connected to a suitable source of biasing voltage supply for the input electrode 28.

The positive terminal of the biasing voltage supply and the negative terminal of the plate voltage supply, are connected to the cathodes 30 of the two thermionic relays 21, 29. Core bodies, 4, 5, are shown in the windings 2, 3, respectively, and the adjustable coupling secured by the means shown in Fig. 1 is indicated by the curved arrow running across the two core bodies. To those skilled in the art, it will be clear that, by suitable adjustment of the condensers, I3, I, and of the coupling between the core bodies 4, 5, and the aawciated coils 2, 31, the two resonant circuits may he adjusted to a desired resonant frequency and ultaneously to a condition oi cptimum Having thus described several illustrative em badlments of my invention, and having shown haw such high-frequency coupling devices are tn be connected into an amplifying system, what ctaim is: 1

3. an intermediate irequency coupling device including two resonant circuits each having an adjustable condenser and a low-loss low-capacity spaced-turn winding having more layers than per layer, a magnetic core within each oi windings, said cores being of commiuuted magnetic material of low permeability and being ccaxialiy disposed and adjusted relatively one to the other to produce a condition of substantially optimum coupling between said circuits.

2. an intermediate-frequency coupling device including two resonant circuits each having an adjustable condenser and a low-loss low-capacity spaced-turn winding having more layers than turns per layer, a magnetic core within each of said windings, said cores being of open type and of comminuted magnetic material and being coaxlally disposed end to end and adjustable relatively one to another to produce a condition of substantially optimum coupling between said circuits.

3. An intermediate frequency coupling device including two resonant circuits each having an adjustable condenser and a low-loss low-capacity spaced-turn winding having more layers than turns per layer, a cylindrical magnetic core with in each of said windings, said cores being of ccni== mlnuted magnetic material of low permeability and being? arranged coaxially to iorm an air gap, the length of said air can being adjusted to produce a ccnditlon oi" substantially optimum coupliug between said circuits.

d. An intermediate frequency coupling device including two resonant circuits each having an adjustable condenser and a low=ioss low canacity spaced-tusn winding having more layers than turns her layer, a magnetic core with n each of said windings, said cores being of comminuted magnetic material having a permeability of the order of iii and being ccairlelly so dispcsed relatively one to the other as to produce a condition of substantially optimum coupling between said circuits.

5. An intermediate frequency coupling device including two resonant circuits each having an adjustable condenser and a low-loss low-capacity spaced-turn winding having more layers than turns per layer, a magnetic core within each of said windings, said cores being of comminuted magnetic material of low permeability and being ooaxially disposed, and means for adjusting one of said cores relatively to the other to produce a condition of substantially optimum coupling between said circuits.

ALFRED CROSSLEY. 

